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Old 08-16-2017, 10:19 PM   #1 (permalink)
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Default Vtx broken spindle bolt

I've had this happen twice, one of the spindle bolts on the Trex 700n dfc sheered off in flight while running vtx blades.

Anyone else having this happen?

Some details, the first time I flew vtx 690's, mid flight one of the bolts snapped in flight and the blades parted. This was the first major crash with the hell and after several hundred flights I never changed the spindle shaft or bolts so I assumed it was poor maintenance.

Forward several months, I've been flying rail 716's governor set to about 2050. Spindle shaft changed as part of maintenance about 60 flights ago. Bolted up the vtx 717, no setup changes made, and three minutes into the first flight the blades ejected again. Spindle bolt broke.

I love the way vtx flies, but I am confident they are causing the bolts to snap. Now don't misunderstand, love the blades and I am not looking to bash, but I am trying to determine what is it about these blades that my heli just doesn't seem to like. My goal is to keep flying the vtx.

I appreciate any input or suggestions.
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Old 08-17-2017, 09:08 AM   #2 (permalink)
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I am not a mechanical engineer nor do I know anything about aerodynamics. But that is certainly interesting.

What is the VTX weight difference from the rail blades?
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Old 08-18-2017, 04:59 AM   #3 (permalink)
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Simply do the maths on centripetal force from the blades vs. spindle bolt tensile strength, should answer the question relatively quickly.
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Old 08-18-2017, 07:31 AM   #4 (permalink)
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Default ahhh, sure

while both of those statements, I'm sure, are true.. not sure they actually help. for example, I do not know the tensile strength of the spindle bolt, nor do I know the centrifical force of those blades spinning at 2050 rpm or the additional force added, if any, of the tic toc maneuver.

reminds me of the old helicopter / Microsoft joke:

A helicopter was flying around above Seattle yesterday when an electrical malfunction disabled all of the aircraft's electronic navigation and communications equipment. Due to the clouds and haze, the pilot could not determine the helicopter's position and course to steer to the airport.
The pilot saw a tall building, flew toward it, circled, drew a handwritten sign, and held it in the helicopter's window. The pilot's sign said, "WHERE AM I?" in large letters.
People in the tall building quickly responded to the aircraft, drew a large sign, and held it in a building window. Their sign said, "YOU ARE IN A HELICOPTER."
The pilot smiled, waved, looked at his map, determined the course to steer to SEATAC airport, and landed safely.
After they were on the ground, the co-pilot asked the pilot how the "YOU ARE IN A HELICOPTER" sign helped determine their position.
The pilot responded "I knew that had to be the Microsoft building because, similar to their help-lines, they gave me a technically correct but completely useless answer."
--------------------------------------

So... I is it safe to conclude no one else has experienced this and I am the only one?
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Old 08-18-2017, 10:49 AM   #5 (permalink)
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In the 13 months we have distributed the VTX blade line from MikadoUSA we've never once heard of this problem.
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Old 08-18-2017, 11:17 AM   #6 (permalink)
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Default hmm ..

So it has been suggested that the Align Trex 700n DFC is not up to the task of my flying style coupled with the advanced blade design of the VTX.

Anyone out there flying this combo?

I am thinking of turning my headspeed down and trying that after the rebuild, but that is a damn expensive test and turning the headspeed down means I may be dropping out of the sweet spot for the motor / pipe, anyone have any thoughts on that.

I want to reiterate I have no thought the blades are faulty or design is poor, just the opposite in fact. I am just trying to determine what is the cause and are these blades safe to fly on that heli? I consider myself fortunate, I was looking for wobble in tic tocs so I was up higher than usual with the disc facing me when it popped and the blades shot out either side several meters (wish it was on video, kinda cool until the parts cost is added). Had I been flying my usual tic toc, low to the ground, kinda in front of me, one of the blades would have made it to the pilot stand (i.e. my chest), or the pit area.

all thoughts welcome.
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Old 08-18-2017, 12:01 PM   #7 (permalink)
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Wow, snapping a 6mm bolt would take a lot strength... Most 700s use 6mm bolts on spindle, so my guess would be that failure is not because of an out of design tensile stress. Maybe you are hitting a resonance where the bolts simply snap. Changing head speed or blade mass would get you out of that resonance, but honestly I dont think is worth a shot. Would be interesting to see the damaged bolts and if other 700n dfc user is running vtxs.

Other thing that comes to mind, is... (And this would be difficult) Could it be that youre overtighting the spindle bolts? I snug my spindle bolts as hard as I can with hex L drivers, and Im pretty sure youll strip the hex head before making damage to the bolt, but are you using a torque wrench or similar to tighten this bolts?
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Old 08-18-2017, 12:06 PM   #8 (permalink)
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Default tightening spindle bolts

I use a standard 'L' shaped hex wrench and one of those fold out hex sets and when I tighten I put some weight into, and use lock tight. I've never felt like I was stressing the 6mm bolt but I have wondered if I did, thinking at the rebuild i'll use less force to tighten the bolts but more lock tight.
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Old 08-18-2017, 01:18 PM   #9 (permalink)
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Lots of people I know of that are using VTX on the ALIGN 700s. Shouldn't make a difference electric or nitro. Though it is odd to me that you're using such huge blades on a nitro. VTX already produce a ton of lift for their size and the 700N is very light. That said, I think there's certainly something very wrong here. No way a standard blade, I don't care how big or heavy or powerful, should be able to snap a M6 bolt. The same head is used on the T-REX 800 and even some people have run T-REX 800 Nitros. I hate to say it, though, I have no clue "what" could be wrong.



There's really no need to go ape on these bolts. Use loctite and let it do its job. That's what it's there for

On nitro, I use red on pretty much everything. You don't need much tightening force, especially on such large bolts where there will be plenty of loctite on them. Just tighten until you feel them seat fully, then just about 1/8 turn more. Beyond this, tightening more doesn't actually do much to add to the strength of the loctite bond, and doesn't make it any less likely to come out.
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Old 08-18-2017, 09:18 PM   #10 (permalink)
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that's what I was hoping to hear. i'll put her back together and give her a whirl
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Old 08-18-2017, 09:25 PM   #11 (permalink)
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I've seen guys run 780 blades with those headspeeds on m6 bolts. Some 700 even have m4 screws. If you want to feel more confident you can go and purchase stronger hardware. Mcmaster Carr has a good selection. Just choose the grade you want. That said the stock screws should be plenty anyway.
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Old 08-18-2017, 09:32 PM   #12 (permalink)
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the general consensus seems to be I had something wrong anyway..

thanks for the input everyone
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Old 08-18-2017, 09:34 PM   #13 (permalink)
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Yeah, I was thinking about that too. The Blade 700 had M4 bolts. Synergy E5 is M5 and lots of people are "super stretching" them and running 700mm blades. So I don't think it's a bolt size issue at all.
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Old 08-19-2017, 07:56 AM   #14 (permalink)
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Could a thrust bearing issue cause added stress on one side of the head block if the blade grip/dampeners/spindle are not allowed to flex properly? Installing the thrust bearing races backwards on one side may be a consideration.

I have made the same build mistake multiple times before. I had a tail blade on backwards, took it off to fix it, and I installed it backwards again. Same with setting tail gyro directions. Sometimes the brain just isn't cooperating.

The VTX blades do seem to have much more bite available, especially right off of center.

My 10 year old T700N with 3G head installed flew about 1.5 gallons of fuel with VTX 697's at IRCHA without a failure, but I found the 3G head could not keep the VTX blades in track when loaded heavily and aggressively. I bolted those same blades on my Synergy N7 and the they flew soooo much better on that helicopter. NO more blade farting, tracking issues, or bogging the engine when aggressive.

For clarification, my Align T700N had top shelf servos, engine, pipe, etc. installed throughout, all with little time on them. I built my Synergy N7 at IRCHA while flying the Align for 5 days at the event. When the Synergy airframe was complete, I stripped the Align T700N and transferred every single servo and other components into the Synergy. Nothing was changed except the airframe. The VTX flew sooo much better on the Synergy.
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Old 08-19-2017, 09:17 AM   #15 (permalink)
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Let's put it like this.

The LOGO 700 has M6 grade 8.8 spindle screws. I did the math, with my VTX 697 it would take 3700rpm to plastically deform them. Not break, just permanently elongate.

I can assure you, it's not the blades causing your issue.
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Old 08-19-2017, 11:13 AM   #16 (permalink)
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Default Rebuilt rebuilt the head

New dampners, new bearings new spindle new spindle bolts fresh Greece plenty of Loctite I was careful not to over tighten.. with any luck she will not blow apart today
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Old 08-21-2017, 05:23 AM   #17 (permalink)
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Default After rebuild

I rebuilt the head, dropped my headspeed down to ~1980 and successfully put 4 flights on it. I assume I had something wrong with the previous setup, but it does seem to be fine.

Thanks for everyone's input
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Old 08-23-2017, 07:00 PM   #18 (permalink)
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Glad to hear it's working fine. I'm curious if when you changed the spindle for maintenance, if you put in new bolts or re-used the old ones. It's possible that tightening these bolts the way you do, multiple times, stressed the bolts to the limit. With the heavier blade you could have taken them past the limits?

Spindles don't wear in any notable way, but any bolt you're applying heavy torque to should be replaced after just 1 or 2 cycles. The head bolts on your car are disposable. They stretch during the torquing procedure, and installing them a second time usually leads to a snapped bolt.
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